OPS OAIHarvest: fix bug introduced before

* Fixes bug introduced in previous OPS commit where all insert
  files were not correctly handled because the wrong extension
  was looked for.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Merge pull request #159 from kaplun/fix-harvest-bug

OPS OAIHarvest: fix bug introduced before